Qamar dar Aqrab refers to the moon being in the zodiacal sign of Scorpio which happens 2-3 days per month (in that the moon circulates through all the zodiacal signs in one lunar month) and can be determined via astronomical software or eyeballs.
Historically, in a number of cultures, this has been considered an unfavourable time, which is probably why it appears in our heritage. There are some narrations on not marrying or starting a journey at this time, but they are not considered strong.
Some people choose to avoid scheduling marriages at this time, and that is fine. It is also fine not to pay attention to it at all. People vary in their belief in these things.
However, it is good not to get too caught up in these things, e.g. if someone gets married at this time, it doesn't mean that it will be a disaster. Sometimes if we feel that something has a bad "omen", it becomes a self-fulfilling prophecy because then we subconsciously do things to make it go badly, and that isn't healthy at all!
If something happens by accident (for instance, conception), there is no need to worry. Allah knows best what He creates and when, and He has his wisdom for it, especially when it comes to bringing life into this world, which is in His hands.
Of course, it doesn't hurt to give sadaqa a few times if you are concerned, but it isn't necessary. Don't overburden yourself with it by making it a requirement!
